Just days after a pair of researchers outwitted major Windows 7 defenses to exploit Internet Explorer (IE) and Firefox, Microsoft said the measures aren't meant to "prevent every attack forever.
At the same time, it defended the security measures, saying they remained an effective way to hinder exploits.
Pete LePage, a product manager with IE's developer division, stood up for DEP (data execution) and ASLR (address space layout randomization), the security features that two hackers sidestepped to win $10,000 each at the high-profile Pwn2Own hacking contest last Wednesday.
"Defense in depth techniques aren't designed to prevent every attack forever, but to instead make it significantly harder to exploit a vulnerability," LePage said, referring to DEP, ASLR and another feature specific to IE, called Protected Mode.
DEP, which Microsoft introduced in 2004 with Windows XP SP2, is designed to prevent attack code from executing in memory not intended for code execution. ASLR, a feature that debuted with Windows Vista three years ago, randomly shuffles the positions of key memory areas, such as the stack, to make it more difficult for hackers to predict whether their attack code will run. Protected Mode, a sandbox-like technology in which IE runs with restricted rights, is designed to reduce the ability of attack code to "escape" from the browser to write, alter or delete data elsewhere on the PC.
All three anti-exploit features are also found in Windows 7, Microsoft's newest operating system.
Peter Vreugdenhil, a freelance vulnerability researcher from the Netherlands, and a German researcher who only goes by his first name of Nils, each bypassed Windows 7's DEP and ASLR when they successfully attacked IE8 and Firefox 3.6, respectively, at the Pwn2Own hacking challenge.
LePage's comments Friday were the first from Microsoft on the DEP and ASLR circumventions since Pwn2Own concluded.
In a post to the Windows Security blog LePage used the analogy of a fire-proof safe, equating its ability to withstand fire and heat with Windows' security. "Without defense in depth techniques, a fire-proof safe may only protect its contents for an hour or two," LePage said. "A stronger fire-proof safe with several defense in depth features still won't guarantee the valuables forever, but adds significant time and protection to how long the contents will last."
Microsoft has made the point before that DEP, ASLR and other defensive measures don't guarantee that Windows can withstand attack. Last summer, for example, Robert Hensing, an engineer in the Microsoft Security Response Center (MSRC), said, "DEP by itself is generally not a robust mitigation."
But in the same post to Microsoft's Security Research & Defense blog|Security Research & Defense : Understanding DEP as a mitigation technology part 1]] , Hensing also said, "DEP and ASLR used together are very difficult to bypass."
Microsoft's LePage said much the same last Friday. "Defense in depth features, including DEP and ASLR, continue to be highly effective protection mechanisms," he claimed.
Vreugdenhil, who exploited IE8 on Windows 7, didn't seem to think they were that difficult to elude. "It took me six or seven days to get everything to work," he said last week in an interview after his Pwn2Own victory. Although he didn't reveal the two exploits he used to bypass DEP and ASLR, then hack Microsoft's browser -- by the rules of the contest, he's not allowed to talk in detail until the flaws are fixed -- Vreugdenhil posted a paper ( download PDF ) where he explained how he evaded Windows 7's protections.
Charlie Miller, another Pwn2Own winner who bypassed DEP and ASLR defenses in Apple Update's Snow Leopard operating system to hack Safari last week, sided with Microsoft on the measures' anti-exploit skills. "It's more difficult [to exploit vulnerabilities] than it was before," Miller said. "Before, any of the 20 bugs I found would have been fine for winning," he continued, referring to his recent discovery of 20 vulnerabilities in Mac OS X, Safari, Microsoft Office and Adobe Reader. "But this year, it took a better bug, a best of breed bug. Now you really need a very special, well-behaved vulnerability [to win]."
Miller is the only researcher to "three-peat" at Pwn2Own, having won prizes in 2008, 2009 and again this year.
One reason why it's more difficult now, Miller said, is because of DEP and ASLR, both of which Mac OS X also uses, though the latter is only partially implemented by Apple's operating system. But he also argued that their presence is a double-edged sword, and a dull one at that.
"With ASLR, exploiting was supposed to be harder, but I still win [at Pwn2Own]," Miller said. "Then they added DEP and I still win. People don't think we need to find bugs now that there's mitigation. But that's not the case. [DEP and ASLR] are just one piece to security, and they don't work as well as people think they will."
In fact, earlier this month, a Google engineer and a former Microsoft security engineer, posted proof-of-concept code for bypassing DEP. At the time, a Microsoft spokesman declined to commit to a revamping of the security feature, instead asserting that any skirting of DEP could not compromise a computer by itself.
Other researchers were confident that the code's publication meant DEP circumvention would increase. "This can be used to further enhance exploits, and I expect that we'll start seeing it being used within exploits fairly soon," said David Sancho, a senior threat researcher at Trend Micro, in an interview four weeks ago.
"Even as security improves, researchers end up winning," Miller said. "We're obviously in better shape [in terms of security], but it's still true that you can't feel totally confident in a full-patched computer."